Filter devices such as are used for swimming pool filtration commonly use diatomaceous earth as a filter media. In a diatomaceous earth filter a plurality of screen or cloth covered filter grids acting in parallel are coated with a layer of diatomaceous earth. Water containing suspended solids is then circulated into the filter housing, through the diatomaceous earth cake and filter grid to an outlet manifold, and ultimately returned clean to the source of the impure water.
Over the course of the filtration process, the filter cake becomes laden with dirt, the pressure drop across the filter increases, and the filtration flow rate decreases. Once the pressure drop becomes unacceptably high, the filter cake must be removed and a new coating of diatomaceous earth applied to the filter cloth or screen surface. Removal of the dirt laden filter cake can be accomplished by disassembling the filter and manually removing the cake (for example, by hosing down the diatomaceous earth covered grids).
It is known that it is advantageous to "regenerate" a diatomaceous earth filter (or more specifically, the filter media therein) rather than completely disassembling, cleaning and recoating the filter grids with a new layer of diatomaceous earth. By "regenerate," it is meant that the dirt laden diatomaceous earth filter cake is, in some manner without disassembling the filter, removed from the surface of the grids, and then allowed to redeposit on the grids to form a regenerated diatomaceous earth layer.
Regeneration can be accomplished, for example, by reversing the flow of liquid through the filter cloth or screen to dislodge the cake within the housing and allowing it to recoat when the filter operation is restarted. By regenerating the filter cake, more efficient use of a single treatment of diatomaceous earth is achieved. After the cake is regenerated a number of times and the contaminant level becomes too great, the diatomaceous earth cake must then be removed by the usual disassembly means. Structures have also been proposed to provide a way to dislodge the cake. For example, U.S. Pat. No. 3,310,175 describes a diatomaceous earth filter which utilizes helical springs surrounding tubular up-flow filter grids wherein the springs can be moved to physically remove dirt laden diatomaceous earth. Another example is U.S. Pat. No. 4,944,887 which describes a diatomaceous earth filter including filter grids disposed in a spiral fashion, which grids can be moved (from the exterior of the filter) by the use of a handle to "bump" and thereby dislodge and regenerate diatomaceous earth from the surface of the filter grids. The '887 patent also describes how the movement of the filter grids creates turbulence within the water to strip the filter cake.
It would be advantageous to have an easily manufactured and maintained structure which could nevertheless achieve the necessary regeneration of the diatomaceous earth filter by combining more than one of these physical or turbulence type cake removal modes.
Thus, previous regenerative diatomaceous earth filter designs have incorporated either a structure for physically dislodging the filter-aid cake or a structure for moving the filter grids to create turbulence, which turbulence is used to dislodge the cake. It is therefore an object of the present invention to achieve a design for a regenerative filter which takes advantage, simultaneously, of both physical dislodging of a filter aid cake, and the use of turbulence to speed and complete the removal of the filter cake.